Methods of capturing and immobilizing radioactive nuclei with metal fluorite-based inorganic materials

ABSTRACT

Methods of capturing and immobilizing radioactive nuclei with metal fluorite-based inorganic materials are described. For example, a method of capturing and immobilizing radioactive nuclei includes flowing a gas stream through an exhaust apparatus. The exhaust apparatus includes a metal fluorite-based inorganic material. The gas stream includes a radioactive species. The radioactive species is removed from the gas stream by adsorbing the radioactive species to the metal fluorite-based inorganic material of the exhaust apparatus.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was developed under Contract DE-AC04-94AL85000 betweenSandia Corporation and the U.S. Department of Energy. The U.S.Government has certain rights in this invention.

TECHNICAL FIELD

Embodiments of the present invention are in the field of radioactivenuclei capture and immobilization, and, more particularly, relate tomethods of capturing and immobilizing radioactive nuclei with metalfluorite-based inorganic materials.

BACKGROUND

One of the great concerns in the nuclear power field throughout theworld is the safe disposal and isolation of used fuels from reactors orwaste streams from reprocessing plants. In particular, entrapment ofhighly mobile radionuclides such as iodine (¹²⁹I) and technetium (⁹⁹Tc)produced from a fission process and subsequent capturing andimmobilization of these radionuclides in an appropriate waste form is agreat technical challenge because of the high mobility of theseradionuclides and the difficulty in incorporating them into any existingwaste forms such as glass, ceramics, and grout. Iodine (¹²⁹I) andtechnetium (⁹⁹Tc) both have long half-lives and can exist as gaseous oranionic species that are highly soluble and poorly sorbed (absorbed oradsorbed) by natural materials. Waste forms are probably the onlyengineered barrier to limit their release into a human-accessibleenvironment after their disposal. Furthermore, a majority (>99%) of ¹²⁹Iwill enter into the dissolver off-gas stream during fuel reprocessing.It is thus highly desirable to develop a material that can effectivelyentrap gaseous iodine during the off-gas treatment.

Thus, further advancements are needed in the area of radioactive nucleicapture and immobilization.

SUMMARY

Embodiments include methods of capturing and immobilizing radioactivenuclei with metal fluorite-based inorganic materials.

In an embodiment, a method of capturing and immobilizing radioactivenuclei includes flowing a gas stream through an exhaust apparatus. Theexhaust apparatus includes a metal fluorite-based inorganic material.The gas stream includes a radioactive species. The radioactive speciesis removed from the gas stream by adsorbing the radioactive species tothe metal fluorite-based inorganic material of the exhaust apparatus.

In another embodiment, a method of synthesizing a metal fluorite-basedinorganic material includes charging a reaction vessel with a solutionhaving an aluminum precursor and a fluorine precursor. A precipitatingagent is added to the solution having the aluminum precursor and thefluorine precursor. An aluminum (Al)-rich fluorite precipitate is thenisolated.

In another embodiment, a nanoporous material is composed of a metalfluorite-based inorganic compound having a surface area greater thanapproximately 1 square meter per gram (m²/g).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a plane view of a 60% fluorinated alumina surfacemodel for use in a Grand Canonical Monte Carlo (GCMC) simulation ofiodine gas adsorption, in accordance with an embodiment of the presentinvention.

FIG. 1B is a plot of iodine adsorption onto unmodified or fluorinatedalumina surfaces predicted from the GCMC simulation, in accordance withan embodiment of the present invention.

FIG. 1C is a plot showing the measurements of surface normalized iodinesorption capability of metal fluorite or hybrid fluorine/oxygen surfacesin comparison with conventional oxide-based adsorbents, in accordancewith an embodiment of the present invention.

FIG. 2 depicts a flowchart representing an exemplary series ofoperations in a method of capturing and immobilizing radioactive nucleiwith a metal fluorite-based inorganic material, in accordance with anembodiment of the present invention.

FIG. 3A illustrates a cross-sectional view of a gas stream carrying aradioactive species and entering an exhaust apparatus charged with ametal fluorite-based inorganic material, corresponding to operation 202of the flowchart of FIG. 2, in accordance with an embodiment of thepresent invention.

FIG. 3B illustrates a cross-sectional view of the gas stream exiting theexhaust apparatus with the radioactive species adsorbed to the metalfluorite-based inorganic material, corresponding to operation 204 of theflowchart of FIG. 2, in accordance with an embodiment of the presentinvention.

FIG. 3C illustrates a cross-sectional view of the gas stream terminatedwith the radioactive species still adsorbed to the metal fluorite-basedinorganic material, corresponding again to operation 204 of theflowchart of FIG. 2, in accordance with an embodiment of the presentinvention.

FIG. 4 depicts a flowchart representing an exemplary series ofoperations in a method of synthesizing a metal fluorite-based inorganicmaterial, in accordance with an embodiment of the present invention.

FIG. 5A illustrates a cross-sectional view of a reaction vessel chargedwith an aluminum precursor and a fluorine precursor for synthesizing ametal fluorite-based inorganic material, corresponding to operation 402of the flowchart of FIG. 4, in accordance with an embodiment of thepresent invention.

FIG. 5B illustrates a cross-sectional view of a reaction vessel chargedwith the aluminum precursor and the fluorine precursor with aprecipitating agent added thereto, corresponding to operation 404 of theflowchart of FIG. 4, in accordance with an embodiment of the presentinvention.

FIG. 5C illustrates a cross-sectional view of a reaction vessel with analuminum (Al)-rich fluorite precipitate therein, in accordance with anembodiment of the present invention.

FIG. 5D illustrates an isolated aluminum (Al)-rich fluorite precipitate,corresponding to operation 406 of the flowchart of FIG. 4, in accordancewith an embodiment of the present invention.

FIG. 6A includes, as a table, a material synthesis matrix for directprecipitation, in accordance with an embodiment of the presentinvention.

FIG. 6B includes a table of material characterization and sorptiontesting for synthesized metal fluorite materials, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Methods of capturing and immobilizing radioactive nuclei with metalfluorite-based inorganic materials are described herein. In thefollowing description, numerous specific details are set forth, such asreagents suitable for aluminum fluorite synthesis, in order to provide athorough understanding of embodiments of the present invention. It willbe apparent to one skilled in the art that embodiments of the presentinvention may be practiced without these specific details. In otherinstances, well-known processing operations, such as sample preparation,are not described in detail in order to not unnecessarily obscureembodiments of the present invention. Furthermore, it is to beunderstood that the various embodiments shown in the figures areillustrative representations and are not necessarily drawn to scale.

Embodiments of the present invention may relate to methods andcompositions useful in capture and possible disposal of radionuclides,particularly highly volatile or mobile radionuclides, as well as otherhazardous materials. One or more embodiments provide a new set of gettermaterials and possible waste forms for entrapping or capturing andimmobilizing radionuclides, especially ¹²⁹I and ⁹⁹Tc, as well as otherhazardous materials.

High performance inorganic solid adsorbents for capturing gaseousradionuclides may be required in multiple process operations of anadvanced fuel cycle. For example, a vented fuel pellet or getter concepthas been proposed to extend fuel burn-ups. Such a concept may beintentionally designed to enhance fission gas release from the fuelpellet and to sequester the gaseous fission products in the fuel'splenum using an adsorbent material. In used fuel reprocessing, effortshave been made to develop getter materials for capturing fission gasesreleased from fuel dissolution and oxidation processes. In anembodiment, such materials are also useful for radionuclide containmentin an event of nuclear plant accident. Furthermore, in an embodiment, atthe backend of the fuel cycle, radionuclide getter materials are alsoutilized for either long-term geologic waste disposal or interim usedfuel storage.

In accordance with an embodiment of the present invention,fluorite-based inorganic solid adsorbents, either natural or synthetic,are used for off-gas treatment. Such adsorbents may perform far betterthan conventional oxide-based materials with respect to gaseousradionuclide sorption. In one such embodiment, the sorption affinity offluorite-based inorganic solid adsorbents for gaseous iodine is 2 to 3orders of magnitude stronger than the conventional oxide-basedadsorbents.

Described herein are chemical precipitation and thermal decompositionroutes for the synthesis of high surface area nanoporous metal fluoritematerials. In an embodiment, the synthesized fluorite materials exhibitexcellent iodine sorption capabilities. Furthermore, these materials mayhave a wide range of applications in advanced nuclear fuel cycles aswell as chemical industries. Given their high chemical stability, theadsorbent materials described in embodiments herein may be ideal for usein extreme chemical and physical environments, for example, in acidicelevated temperature environments.

The sorption capability of an adsorbent material may, to a large extent,be determined by the interaction of an adsorbate with the surfaceterminating atoms of the material. Conventional solid inorganicadsorbents for radionuclide capture are exclusively based on oxidecompounds, in which the surface terminating atoms are overwhelminglyoxygen atoms. Using a Grand Canonical Monte Carlo (GCMC) simulation, inaccordance with an embodiment of the present invention, we hereindescribe that replacing (at least conceptually) surface oxygen atomswith fluorine on γ-aluminum oxide significantly improves the capabilityof the material for iodine sorption (see, e.g., description of FIG. 1Bbelow). Such improvement may be due to the additional polarization ofiodine molecules induced by fluorine atoms. The simulation results areconsistent with actual experimental data presented below. In anembodiment, based both the simulated and the experimental data, wedescribe herein that non-oxide materials can be used as adsorbents forgaseous radionuclides, with sorption capabilities significantly improvedover conventional oxide-based adsorbents.

FIGS. 1A and 1B depict a Grand Canonical Monte Carlo (GCMC) simulationof iodine gas adsorption on a modified γ-alumina (Al₂O₃) surface, inaccordance with an embodiment of the present invention. Specifically,FIG. 1A illustrates a plane view of a 60% fluorinated alumina surfacemodel 100 for use in a GCMC simulation of iodine gas adsorption. Theballs with the lightest shading represent fluorine (F) atoms, the ballswith the darkest shading represent oxygen (O) atoms, and the balls withthe intermediate shading represent aluminum (Al) atoms. FIG. 1B is aplot 102 of iodine adsorption onto unmodified (line 104) or fluorinated(line 106) alumina surfaces predicted from the GCMC simulation.

As mentioned briefly above, and in accordance with one or moreembodiments of the present invention, non-oxide materials (e.g.,materials with fluorinated surfaces) show marked improvement versusconventional oxide-based materials with respect to radionuclidesorption. In one such embodiment, the sorption capability of a fluoritematerial for gaseous iodine (as normalized to surface area) isapproximately 1000 times greater than that for conventional oxidematerials. As an example, FIG. 1C is a plot 110 showing the measurementsof surface normalized iodine sorption capability of metal fluorite(e.g., CaF) or hybrid fluorine/oxygen surfaces (e.g., Al—O—F) incomparison with conventional oxide-based adsorbents (e.g., Al—O), inaccordance with an embodiment of the present invention. In considerationof plot 110, in an embodiment, one or both of two phenomena dictate themarked improvement of adsorption: (a) an increased surface area(although surface area is normalized in plot 110) and (b) binding energy(e.g., increased affinity using F-terminating atoms in place ofO-terminating atoms).

In an embodiment materials suitable for improved iodine (or otherradionuclide) adsorption are synthesized in consideration of one or bothof two key factors: (1) the demonstration described herein of highsorption affinity of metal fluorites for iodine-129 (e.g., as shown inplot 110 of FIG. 1C), and (2) techniques described herein forsynthesizing high surface area nanoporous materials. In one suchembodiment, the combination of both high sorption affinity and highspecific surface area renders the resulting nanoporous metal fluoritematerials as excellent radionuclide adsorbents.

In an aspect of the present invention, radioactive nuclei are capturedand immobilized with a fluorite-based material. For example, FIG. 2depicts a flowchart 200 representing an exemplary series of operationsin a method of capturing and immobilizing radioactive nuclei with ametal fluorite-based inorganic material, in accordance with anembodiment of the present invention. FIGS. 3A-3C illustrate schematicrepresentations of various operations of the flowchart of FIG. 2, alsoin accordance with an embodiment of the present invention.

Referring to operation 202 of flowchart 200, and to corresponding FIG.3A, a method of capturing and immobilizing radioactive nuclei with ametal fluorite-based inorganic material includes flowing a gas streamthrough an exhaust apparatus including or charged with a metalfluorite-based inorganic material (either natural or synthetic), the gasstream carrying a radioactive species. In a specific example, a gasstream 302 carrying a radioactive species 304 is flowed through anexhaust apparatus 306 charged with a metal fluorite-based inorganicmaterial 308.

In an embodiment, flowing the gas stream 302 through the exhaustapparatus 306 includes flowing through a nanoporous metal fluorite-basedinorganic material (e.g., a nanoporous example of material 308). In anembodiment, flowing the gas stream 302 through the exhaust apparatus 306includes flowing through a natural or a synthetic aluminum (Al)-richfluorite (e.g., examples of material 308). In one such embodiment, theflowing is performed through a synthetic aluminum (Al)-rich fluoritewith a surface area greater than approximately 1 square meter per gram(m²/g). In an embodiment, a natural fluorite material, such as naturallyoccurring CaF₂, is used.

Referring to operation 204 of flowchart 200, and to corresponding FIG.3B, the method also includes removing, from the gas stream, theradioactive species by adsorbing the radioactive species to the metalfluorite-based inorganic material of the exhaust apparatus. In aspecific example, the gas stream 302′ exits the exhaust apparatus 306with the radioactive species 304 adsorbed to the metal fluorite-basedinorganic material 308. In an embodiment, removing the radioactivespecies 304 from the gas stream 302 includes removing a nuclei such as,but not limited to, ¹²⁹I.

Referring to FIG. 3C, the gas stream 302/302′ is terminated with theradioactive species 304 still adsorbed to the metal fluorite-basedinorganic material 308 in the exhaust apparatus 306. In one suchembodiment, referring to optional operation 206 of flowchart 200, themethod further includes disposing of the metal fluorite-based inorganicmaterial 308 having the radioactive species 304 adsorbed thereon. Inanother such embodiment, referring to optional operation 208 offlowchart 200, the method further includes releasing the radioactivespecies 304 from the metal fluorite-based inorganic material 308. Forexample, in a specific embodiment, the radioactive species 304 isreleased from the metal fluorite-based inorganic material 308 into amedium such as, but not limited to, a second gas stream, a liquidextraction medium, or a solid extraction medium for chemical separationor concentration.

In an aspect of the present invention, a nanoporous-structured materialmay be engineered through thermal decomposition or direct precipitation(e.g., by sol-gel methods). For example, FIG. 4 depicts a flowchart 400representing an exemplary series of operations in a method ofsynthesizing a metal fluorite-based inorganic material, in accordancewith an embodiment of the present invention. FIGS. 5A-5D illustrateschematic representations of various operations of the flowchart of FIG.4, also in accordance with an embodiment of the present invention.

Referring to operation 402 of flowchart 400, and to corresponding FIG.5A, a method of synthesizing a metal fluorite-based inorganic materialincludes charging a reaction vessel with a solution including analuminum precursor and a fluorine precursor. Specifically, a reactionvessel 500 is charged with an aluminum precursor 502 and a fluorineprecursor 504, e.g., in the form of a solution 506.

In an embodiment, charging the reaction vessel 500 with the solution 506including the aluminum precursor 502 and the fluorine precursor 504includes charging the reaction vessel 500 with aluminum trichloride(AlCl₃) and a trifluoracetate. In one such embodiment, thetrifluoracetate is a neutralized product of trifluoracetic acid. In oneembodiment, charging the reaction vessel 500 with AlCl₃ and thetrifluoracetate further includes using a water/ethanol solution 506. Inan embodiment, charging the reaction vessel 500 with the solutionincluding the aluminum precursor and the fluorine precursor includesusing an aluminum:fluorine molar ratio of approximately 1:1 or 1:3,examples of which are described in more detail below.

Referring to operation 404 of flowchart 400, and to corresponding FIG.5B, the method also includes adding a precipitating agent to thesolution including the aluminum precursor and the fluorine precursor.Specifically, the reaction vessel 500 charged with the aluminumprecursor 502 and the fluorine precursor 504 has a precipitating agent508 added thereto.

In an embodiment, adding the precipitating agent 508 includes addingpolyethylene oxide. Upon adding the precipitating agent 508, referringto FIG. 5C, the reaction vessel 500 has an aluminum (Al)-rich fluorite510 precipitated therein.

Referring to operation 406 of flowchart 400, and to corresponding FIG.5D, the method also includes isolating an aluminum (Al)-rich fluoriteprecipitate. Specifically, an isolated aluminum (Al)-rich fluoriteprecipitate 512 may be collected on a filter 514.

In an embodiment, isolating the Al-rich fluorite precipitate 512includes collecting the Al-rich fluorite precipitate 510 of FIG. 5C onthe filter 514, subsequently washing the Al-rich fluorite precipitatewith deionized water, and subsequently heating the Al-rich fluoriteprecipitate. In one such embodiment, heating the Al-rich fluoriteprecipitate 510 of FIG. 5C dehydrates the Al-rich fluorite precipitateto provide the isolated aluminum (Al)-rich fluorite precipitate 512.

In an embodiment, referring to optional operation 408 of flowchart 400,the method may also include, prior to adding the precipitating agent508, adding a block co-polymer to the reaction vessel 500. In one suchembodiment, isolating the Al-rich fluorite precipitate 512 includesthermalizing the precipitate 510 of FIG. 5C (which, in this embodiment,would include the block co-polymer encapsulated therein) to remove theblock co-polymer. In an embodiment, the block co-polymer is used tofabricate porosity (and hence increased surface area) into a syntheticmetal fluorite-based inorganic material. In a specific embodiment, theblock co-polymer is a material such as, but not limited to poly(ethyleneglycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), knowncommercially as P123.

As mentioned briefly above, thermal decomposition may be used as part ofthe fabrication or synthesis of a nanoporous aluminum fluorite material.In an embodiment, such a process is based on the initial precipitationof an Al—OOCCF₃ precursor (e.g., may be the precipitate 510 of FIG. 5C)disposed around an organic polymer framework, e.g., such as a blockco-polymer framework. In one such embodiment, the precursor isdecomposed at approximately 350° C. Subsequently, the polymer is removedvia calcination at approximately 550° C. The end result may be to leavebehind a fluoride rich, high surface area Al-based precipitate.

In specific set of experimental embodiments, solutions for Products 1-5below were all prepared on a hot plate set at approximately 60° C. withconstant stirring. Approximately 12 grams of a block co-polymer wasdissolved in approximately 72 milliliters of ethanol (EtOH) (Solution 1)to provide a nanoporous template. Approximately 34 grams of aluminumtrichloride (AlCl₃) was dissolved in approximately 45 milliliters of 1:1EtOH:H₂O solution (Solution 2). Solution 2 was added to Solution 1 toprovide a mixed Solution 3. Approximately 40 grams of polyethylene oxide(PO) was added to Solution 3. The precipitate was aged in anapproximately 70-90° C. oven overnight, or longer as necessary forsolvent removal.

To the above basic operations, trifluoroacetic acid (3FAc) was added toprovide a stoichiometric ratio of Al:F of approximately 1:3, and wasadded at various stages and in various forms to provide Products 1-5.Product 1: un-neutralized 3FAc was added to Solution 1. Product 2:neutralized 3FAc (with a pH of approximately 5) was added to Solution 1.Product 3: neutralized 3FAc was added to Solution 3. Product 4:neutralized 3FAc was added to Solution 2 (with an immediateprecipitation operation and no other operations following). Product 5:identical to Product 2, but the PO was not added, and the precipitationproceeded via solvent evaporation in an oven. Once synthesized, allProducts 1-5 were subjected to heating in a muffle furnace forapproximately 2 hours at approximately 350° C., then for approximately 2hours at approximately 600° C.

As also mentioned briefly above, direct precipitation may be used aspart of the fabrication or synthesis of a nanoporous aluminum fluoritematerial. In an embodiment, direct precipitation is based on making anAl—F precursor solution, followed by precipitation of an Al—F richcomplex. The precipitation results from solvent insolubility with andwithout a polymer framework.

In specific set of experimental embodiments, precursor solutions forProducts 6-12 below were all prepared by dissolving approximately 50grams of AlCl₃ in an approximately 70:30 EtOH:H₂O mixture (e.g.,approximately 100 milliliters) and heated to near boiling. Eitherapproximately 8 grams (Al:F=1:1), or approximately 35 grams (Al:F=1:3)of sodium fluoride (NaF) was dissolved in water (e.g., approximately 200milliliters) and a sufficient amount of concentrated hydrochloric acid(HCl) to provide a pH less than approximately 2. In both cases, a slurrywas created. The NaF slurry was then combined with the AlCl₃ solution.The entire solution was then brought to a boil. Upon clarification ofthe solution, the solution was reduced in volume until precipitatesbegan to form. A relatively small amount of water was added tore-dissolve the precipitates. The resulting parent solution was pouredinto various mixtures of EtOH and H₂O. In all but one of the abovemixtures, approximately 16 grams of block co-polymer had been added. Theabove process provided Products 6-12 listed in table 600 of FIG. 6A(table 600 is a material synthesis matrix for direct precipitation). Allprecipitates were then collected through solvent evaporation, dried inan approximately 90° C. oven overnight and subsequently calcined atapproximately 600° C. for approximately 4 hours.

FIG. 6B includes a table 602 of material characterization and sorptiontesting for Products 1-12 (synthesized metal fluorite materials) fromabove, in accordance with an embodiment of the present invention.Referring to table 602, all samples were characterized using BET surfacearea measurements (S. Brunauer, P. H. Emmett, and E. Teller, J. Am.Chem., 1938, 60, 309), which is a surface area technique based onmonolayer adsorption of nitrogen gas, and gaseous iodine sorptionstudies. Results for a natural CaF₂ mineral are also provided as a pointof reference.

These results tabulated in table 602 exemplify the importance of boththe ordering of the operations of the synthesis as well as the solventchoice. In an embodiment, for the thermal decomposition materials(Products 1-5), there is a clear advantage to adding the neutralized3FAc to the parent Al-polymer solution. Adding the neutralized 3FAc atother points provides significantly impacted results. In anotherembodiment, for the direct precipitation materials (Products 6-12),there is a clear advantage to using pure ethanol to precipitate thematerials. For example, Products 6, 8, 9, and 10 all perform better thanthe CaF₂ reference material, which is a natural CaF₂, and all wereprepared in ethanol. Other variables (such as, but not limited to,presence of polymer, stoichiometric ratios, temperature) play only asecondary role. It appears that the addition of fluorine in the varioussynthesis procedures increases the chemical affinity of iodine to thesurface of the resulting synthetic compound. Despite the lower surfaceareas relative to the Al-O material, the fluorinated compounds stilloutperform existing oxide-based adsorbents in total iodine removal.

As mentioned above, the high performance inorganic solid adsorbentsdescribed herein may be applied to capturing gaseous radionuclides inmultiple process operations of an advanced fuel cycle from novel fuelconcepts, to off-gas treatment of used fuel reprocessing, and to nuclearwaste disposal. The applications may potentially be extended to thecapture, separation, and immobilization of non-radioactive chemicals.Given their high chemical stability, the adsorbent materials describedherein can, in an embodiment, be ideal for use in extreme chemical andphysical environments, for example, in acidic elevated temperatureenvironments.

Although a variety of surface areas may be fabricated for analuminum-rich fluorite species and may be suitable for radionuclidecapture, investigations thus far indicate that the greater the surfacearea, the greater the immobilization ability. In an embodiment, ananoporous material is composed of a metal fluorite-based inorganiccompound having a surface area greater than approximately 1 square meterper gram (m²/g). In one such embodiment, the metal fluorite-basedinorganic compound is an aluminum (Al)-rich fluorite species. In anothersuch embodiment, the surface area is greater than approximately 5 squaremeters per gram (m²/g). In yet another such embodiment, the surface areais approximately in the range of 5-100 square meters per gram (m²/g).

Embodiments of the present invention may be based on the capture andencapsulation of radionuclides, particularly highly mobileradionuclides, including radioactive isotopes of iodine and technecium.For example, in one embodiment, methods described herein are used torender hazardous materials less dangerous. Embodiments may also includegenerating getter materials and the resultant waste forms, andcompositions of matter of the getter materials and the resultant wasteforms.

Overall, then, metal fluorite inorganic materials, such as nanoporousaluminum fluorite, represent a class of highly efficient materials foruptake of gaseous iodine (and other volatile radionuclides) even at anelevated temperature. Such materials may, in an embodiment, be convertedto a durable waste form in which the sorbed iodine is effectivelyencapsulated. The technology of the invention may be applied to thetreatment of other radionuclides and even non-radioactive hazardousmaterials. Other radionuclides for which the present invention may beuseful include, but are not limited to, iodine (including ¹²⁹I) (both ingaseous and anionic forms such as I⁻ and IO₃ ⁻), technetium (including⁹⁹Tc) (e.g., in anionic forms such as TcO₄ ⁻), and radioactive isotopesof Pu, Am, U, Th, Np, Se, Cs, Sr, C, CI, H, Xe and Kr in the forms ofmolecular species, cations and anions. Non-radioactive hazardousmaterials that may be disposed of according to one or more embodimentsof the invention include heavy metals such as Pb and Cd in the form ofcations. It is to be understood that metal fluorite species suitable forcapturing and immobilizing such ions may include metal fluorite speciesother than the aluminum rich versions described in details herein; andthey may, in one or more embodiments, include the fluorite compounds ofalkaline earth metals (e.g., Ca), transition metals (e.g., Mn, Zr, Re),or rare earth metals.

Thus, methods of capturing and immobilizing radioactive nuclei withmetal fluorite-based inorganic materials have been disclosed. Inaccordance with an embodiment of the present invention, a method ofcapturing and immobilizing radioactive nuclei includes flowing a gasstream through an exhaust apparatus. The exhaust apparatus includes ametal fluorite-based inorganic material. The gas stream includes aradioactive species. The radioactive species is removed from the gasstream by adsorbing the radioactive species to the metal fluorite-basedinorganic material of the exhaust apparatus. In one embodiment, themethod further includes disposing of the metal fluorite-based inorganicmaterial having the radioactive species adsorbed thereon. In oneembodiment, the method further includes releasing the radioactivespecies from the metal fluorite-based inorganic material into a mediumsuch as, but not limited to, a second gas stream, a liquid extractionmedium, or a solid extraction medium for chemical separation orconcentration.

What is claimed is:
 1. A method of capturing and immobilizingradioactive nuclei, the method comprising: flowing a gas stream throughan exhaust apparatus comprising a metal fluorite-based inorganicmaterial, the gas stream comprising a radioactive species; and removing,from the gas stream, the radioactive species by adsorbing theradioactive species to the metal fluorite-based inorganic material ofthe exhaust apparatus.
 2. The method of claim 1, further comprising:disposing of the metal fluorite-based inorganic material having theradioactive species adsorbed thereon.
 3. The method of claim 1, furthercomprising: releasing the radioactive species from the metalfluorite-based inorganic material into a medium selected from the groupconsisting of a second gas stream, a liquid extraction medium, and asolid extraction medium for chemical separation or concentration.
 4. Themethod of claim 1, wherein flowing the gas stream through the exhaustapparatus comprises flowing through a nanoporous metal fluorite-basedinorganic material.
 5. The method of claim 1, wherein flowing the gasstream through the exhaust apparatus comprises flowing through a naturalor a synthetic aluminum (Al)-rich fluorite.
 6. The method of claim 5,wherein the flowing is performed through a synthetic aluminum (Al)-richfluorite with a surface area greater than approximately 1 square meterper gram (m²/g).
 7. The method of claim 1, wherein removing theradioactive species from the gas stream comprises removing ¹²⁹I nuclei.